Large scale high speed multiplexed optical fiber sensor network

ABSTRACT

A digital spatial and wavelength domain system for multiplexing fiber Bragg grating sensors comprises: a plurality of optical fibers, each including a plurality of fiber Bragg gratings therein, each fiber Bragg grating having a selective center wavelength that is variable in accordance with strain for reflecting or transmitting light at the corresponding center wavelength in accordance with the strain thereat; means for providing broad band illumination ( 110 ) for the fiber Bragg gratings; means for each optical fiber for carrying the light to a selected location; and a wavelength dispersion device ( 136 ) responsive to the light from each of the fibers for wavelength separating the light in each fiber into the center wavelengths in accordance with the location of each fiber so that the selected location of each fiber and the wavelength separated light provides spatially independent signals for each fiber Bragg grating in each optical fiber.

This Application claims the benefit of No. 60/056,560, filed Aug. 19,1997.

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for multiplexingsignals. More particularly, the invention relates to a method andapparatus for de-multiplexing optical signals in the spatial andwavelength domains employing a dispersion device optically coupled to arandom access two dimensional imager and employing software forsub-pixel interpolation.

Optical fiber sensor systems employ multiplexing techniques to allow thesharing of a source and processing electronics to reduce the per sensorcost and thereby improve the competitiveness of such systems. Inaddition, component sharing helps to reduce the overall weight of thesystem and enhances robustness. A variety of multiplexing technologiesare known including spatial, wavelength, frequency and coherence domainmultiplexing. However, the multiplexing capacity of any of thesetechniques is generally limited to about ten sensors due to variousfactors including speed, cross talk, signal to noise ratio andwavelength bandwidth. Some systems employ two or more techniques toincrease multiplexing capacity. In particular spatial domainmultiplexing is advantageously combined with other techniques, generallybecause it does not degrade system performance.

Fiber optic Bragg gratings (FBG) have become one of the most successfulof the optical fiber sensors available. These devices are generallycompact, have absolute wavelength encoding, and have the potential formass production. Sensor signals may be wavelength encoded rather thanintensity encoded. Thus the sensed signal is independent of powervariations in the light source and system losses. Additionally, an arrayof FBG sensors can be readily made by connecting several FBGs havingdifferent center wavelengths in a line along a length of fiber. Each FBGmay be individually addressed using wavelength multiplexing in thewavelength domain. However, wavelength domain alone can only accommodatea relatively small number of FBGs, because a broad band source opticalfibers has only a limited bandwidth. Accordingly, it is desirable toemploy combined multiplexing techniques to increase the capacity of thesystem.

Conventional spatial multiplexing locates sensors into many fiberchannels and may employ a separate electronic signal processing unit foreach channel. Such a system may be improved by using an optical fiberswitch as a special case of spatial multiplexing, thereby allowingmultiple fiber channels to share a single processing unit. However, thespeed of the system, measured as the sample rate of each sensor isconsiderably reduced because of the optical switch, for example, 60 FBGsat a sample rate of 1 Hz. Certain applications such as monitoringaerospace structures or process control and massive data collectionrequire higher multiplexing capacity, and particularly, a highersampling rate are desirable.

A digital space and wavelength domain multiplexing technique, employingmultiple fiber channels, sharing a processing unit, has been reported bythe inventors herein. Single channel systems with multiple FBGs employ adispersion device and line scan camera. The system is only a singledomain device and the system is limited to one dimension and thereforeit can only address a limited number of sensors.

SUMMARY OF THE INVENTION

The present invention seeks to overcome and obviate the disadvantagesand limitations of the described prior arrangements. In particular, theinvention is based upon the discovery that a large scale, high speedoptical fiber sensor network may be provided which has wavelength andspatial multiplexing using a dispersion device an a two dimensional (2D)image sensor to distinguish a plurality of fiber channels on one axisand FBG wavelengths along another. An exemplary embodiment employs arandom access 2D imaging device and a sub-pixel interpolation algorithmfor resolution enhancement.

In an exemplary embodiment, the invention comprises the broad bandsource, a coupler for distributing the source to multiple fiber channelseach including a plurality of fiber brag gratings (FBGs) therealong. TheFBGs in each fiber each have a different center wavelength and thereflected signals from each FBG are carried by a down-lead fibers. Thedown-lead fibers are arranged along a line in a 1D array at the inputport of a wavelength selective dispersion device. The reflected light ispassed through the dispersion device which separates the reflected lightby wavelength and directs the light to a 2D solid state image sensor. Inthe exemplary embodiment the 2D sensor is a random access device tothereby improve data acquisition speed. A sub-pixel interpolationalgorithm is employed to enhance resolution.

In another embodiment the wavelength selective dispersion device, the 2Drandom access imaging device, and an in-line fiber optic input array maybe combined as a module. A broad band source and a coupler fordistribution to multiple fiber channels may be another module. Themodules may be operated separately or combined in a single unit.

The FBG sensor is generally sensitive to both temperature and strain. Inaccordance with the invention. FBG arrays may be produced withtemperature and strain sensitivity combined or separated. FBS array mayalso be adapted to sense physical characteristics which may be readilyconverted to a strain measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings, wherein

FIG. 1 is a generalized illustration of a wavelength and spatial domainmultiplexing device according to the present invention:

FIG. 1A is a detail of a 1D fiber output array;

FIG. 1B is a schematic illustration of a random access image sensor anddriver;

FIG. 1C is a fragmenting illustration of a portion of the image sensorillustrating the output spot and pixels;

FIG. 1D is a plot illustrating a weighted algorithm in linear andlogarithmic form;

FIG. 1E is a generalized illustration of the operation of a fiber Bragggrating;

FIG. 2 is a more specific illustration of an apparatus for achievingspatial and wavelength domain multiplexing in accordance with thepresent invention;

FIG. 3 is an embodiment illustrating a transmission type array:

FIG. 4A is an illustration of a strain and temperature separated sensoron the end of a fiber;

FIG. 4B is a schematic illustration of a strain and temperatureseparated inline sensors;

DESCRIPTION OF THE INVENTION

FIG. 1 generally illustrates the operative principle of the presentinvention in which a fiber optic array 12 of channels 14-1 . . . 14-nhaving a plurality of FBG sensors 16-1 . . . 16-m produce outputs 18-1 .. . 18-n. The light in each output is a signal containing a plurality ofdiscrete wavelengths centered at selected wavelengths corresponding tothe center wavelengths of the respective FBGs 16-1 . . . 16-m. The fiberoptic channels 14-1 . . . 14-n are disposed side by side and spatiallyseparated along a line L as shown in FIG. 1A. The likewise spatiallyseparated light outputs 18-1 . . . 18-n are directed at the dispersiondevice 20 which separates each signal into a plurality of correspondingwavelength separated signals 22-1 . . . 22-m for each fiber, whichsignals are directed towards 2D image sensor 24 and which form spots25-1 . . . 25-n thereon.

The sensor 24 is a two dimensional (2D) image sensor 24 having k columns26 and j rows 28 of pixels 30 formed therein where k & j are much largerthan m & n so that a spot 25 falls on a cluster of pixels 30. As can beseen in FIG. 1B, the wavelength separated spots 25-1 . . . 25-n in eachchannel fall more or less into the various columns 26-1 . . . 26-m ofthe image sensor 24 and cover a cluster of pixels 30 along a row 28-1corresponding to the location of the first fiber optic channel 14-1 inthe array 12. Likewise, the rows 28-2 . . . 28-n correspond to thepositionment of the respective fiber optic channels 14-2 . . . 14-nrespectively. The columns 26 represent wavelengths. For example, thesignal 18-1 is broken up into wavelengths 22-1 . . . 22-m correspondingto the number of FBG and form spots near the columns 26-1 . . . 26-mcovering clusters of pixels 30 therein as shown. Each unique pixelcoordinate (xj, yk) on the sensor 24 provides information about thecorresponding signal carried by the fiber optic array 12 the weightedcenter of the light falling on the pixels 30 under each spot 25 is amember of its wavelength and thus provides a strain measurement.

In the exemplary embodiment of the invention shown in FIGS. 1, 1B, & 1Cthe sensor 24 may be a randomly accessible device such as a CMOS imagerwhich allows any selected pixel 30 (xj, yk) or a cluster of pixels to berandomly addressed and read out as opposed to a system which requiressequential scanning of each pixel on the entire imager.

As shown in FIGS. 1B & 1C, light signals 22-1 . . . 22-m appear as spots25 on the image sensor 24 covering more or less pixel clusters 30, alongrows and columns as shown. A driver device 34 which mav be suitablydriven by a programmed computer or microprocessor 36 selectively readsdata from the x,y coordinates of the sensor by selectively addressingthe pixel cluster 30 located at or near the spot locations. Informationas to the position of the spot 25 relative to each of the correspondingproximate pixel clusters 30 may be processed to determine the precisecentral location or centroid 38 at (xj,yk) of the spot 30 in the pixelarray. The location of the spot 25 may be accurately determined bysub-pixel interpolation. In an exemplary embodiment, the centroid 38shown as a cross in FIGS. 1B & 1C, is the weighted average of theilluminated pixels under spot 25. The technique of sub-pixelinterpolation using known algorithms allows for resolution at asub-pixel level and thus provides highly accurate measurements ofstrain. The centroid interpolation technique determines the centroid 38which represents the weighted average of the spot 25 over the pixels 30.FIG. 1D graphically shows centroid interpolation for FIG. 1C. Otheralgorithms include curve fitting; and linear or higher orderinterpolation.

In accordance with the invention, the wavelength of the variouscomponents making up the light 18-1 . . . 18-n represents a measuredparameter. For example. FIG. 1E shows a broad band source S coupled toan optical fiber 40 having m fiber Bragg gratings 16-1 . . . 16-m. Eachhas a corresponding pitch Λ-1 . . . Λ-m developed as a change in therefractive index of the core 42. The pitch is related to a correspondingcenter wavelength λ-1 . . . λ-m is proportional to the pitch Λ-1 . . .Λ-m respectively. As the strain on the FBG 16-1 . . . 16-m changes, thepitch Λ-1 . . . Λ-m likewise changes causing the center wavelength ofthe corresponding reflected signal 22-1 . . . 22-n to changeaccordingly. A change in the wavelength is reflected as a slight shiftin the position of the spot in the sensor corresponding to the change inthe pitch of the FBG. It should be understood that temperature also canchange the pitch and thus the wavelength.

If a plurality of FBGs 16-1 . . . 16-m are formed in the core 42 of thefiber 40, multiple parameters may be sensed using the same fiber tocarry plurality of signals. The problem, of course, as noted above, isto separate the various reflected signals 22-1 . . . 22-n using themultiplexing techniques of the present invention. As the wavelengthchanges, the dispersion device 20 causes a shift in the column positionof the wavelength separated signal which corresponds to an indication ofincreasing or decreasing strain. For each of the spots 25-1 . . . 25-min each row 28-1 . . . 28-n on the sensor 24 a unique strain measurementmay thus be obtained.

FIG. 2 illustrates in further detail an exemplary embodiment of a largescale, high speed optical fiber sensor network 100 in accordance withthe invention. The system includes a broad band light source 110. Lightfrom the source is coupled by a lead fiber 114 to a star coupler 116.The light is split to feed a plurality of single mode fibers 18-1 . . .118-n, one for each channel. Each fiber has a plurality of fiber Bragggratings (FBGs) 120-1 . . . 120-m, each with the predetermined differentcentral wavelength λ1 . . . λm respectively. In the arrangementillustrated, the end of each fiber has a compensating temperature sensor122-1 . . . 122-n. Light reflected by the FBGs in each channel iscoupled to down-lead fibers 125-1 . . . 125-n by a coupler 126-1 . . .126-n in each channel. The free ends 128-1 . . . 128-n of the fibers 118are arranged in a linear fiber bundle array 129 along line L. See, forexample, the end view of the fiber array 12 in FIG. 1A in which thefibers are arranged side by side along line L. Output light 130-1 . . .130-n from each the corresponding fiber end is directed at a dispersionunit 134 which comprises a mirror lens 136 formed with a grooved grating138 on the reflective surface as shown. Grooves 140 in the grating arearranged parallel to the line L of the fiber end faces.

The mirror 136 can form an image of the fiber array on an image plane Pas shown. A solid state image sensor 150 comprising an j by k array ofpixels 152 is disposed in the image plane P as shown. Pixel rows 154correspond to the position of the channels established by fiber ends128-1 . . . 128-n along the line L. The pixel columns 156 correspond tothe number of fiber gratings FBG1 . . . FBGn in the correspondingwavelengths λ1-λn.

The sensor 150 is positioned in such a way that the pixel columns 156 (yaxis) are parallel to the grooves 140 in the grating 138 and to the lineL of the fiber end faces 128-1 . . . 128-n. In addition, the surface ofthe sensor 150 coincides with the image plane P of the mirror lens 136.

The detector 150 has an output 158 which is connected to an interfacecircuit 160 for processing by a microprocessor 162. It should beunderstood that the grating and lens is one of a variety of possibledispersion devices which may be employed.

In accordance with the invention, n fiber channels and m FBGs ofdifferent wavelengths along each fiber form an n by m matrix of brightspots 164 on the detector array 150. Each column 156 in the matrixrepresents the FBGs of the same or similar wavelength in different fiberchannels, and each row represents different FBGs along the same fiber.In other words, the spatial positions of the fiber channels are encodedonto the position along the y axis of the detector while the wavelengthsare encoded along the x axis. The precise central wavelength of an FBGin a particular channel can therefore be detected by locating the exactposition of the associated spot 164 along the x axis. The resolution ofmeasurement depends upon the spatial resolution of the dispersion deviceand the detector array. The output of a spectrum analyzer is generallyregarded to have too low a spatial resolution to meet the requirementsfor FBG based systems. However, in accordance with the invention,resolution can be greatly improved by employing any one of a number ofknown digital interpolation algorithms so that a strain measurementresolution to near micro strain may be achieved.

According to the invention, the FBGs are usually fabricated usingholographic or phase mask techniques to expose a germanium doped (andsometimes boron co-doped) optical fiber to a periodic intensitydistribution. These fibers are photosensitive, meaning that theirrefractive indices change when they are exposed to UV light. Because ofthis photosensitivity, the impinging sinusoidal intensity distributionresults in a sinusoidal refractive index distribution in the fiber core.The combined effect of the periodic index distribution is to reflectlight at a very specific wavelength known as the “Bragg wavelength”.This wavelength is predictable in terms of the mean refractive index, η,and the pitch of the periodicity, Λ, by λ_(B)=2ηΛ. Sensors are made fromthese gratings by taking advantage of the fact that the grating pitchand refractive index are both functionally dependent on strain.Therefore strain on the grating causes the Bragg wavelength to shiftleft or right. The wavelength encoded nature of FBGs offers the greatestpotential for multiplexing in wavelength domain along a single length ofoptical fiber. Multiplexing is accomplished by producing an opticalfiber with a sequence of spatially separated Bragg gratings, each havinga different pitch, Λ_(k), k=1, 2, 3, . . . n. The resulting Braggwave-lengths associated with each pitch are therefore given byλ_(Bk)=2ηΛ_(k), k=1, 2, 3, . . . n. Because the unstrained Braggwavelength of each FBG is different, the information from each sensor isindividually determined by examining the wavelength spectrum. Forexample, where a strain field at grating 16-2 (124-2) is uniquelyencoded as a perturbation to Bragg wavelength 12. The Bragg wavelengthsassociated with the other gratings remain unchanged.

FBGs are the natural sensor of tensile strain when they are attached onor embedded in the host material. However. FBGs can be adapted to detecta wide range of other physical parameters by converting the change ofthe relevant parameter into strain. For example. FBGs can be used tomeasure humility by coating the FBG with a layer of hydrogel, whichexpands upon water absorption thus converting humility into strain.Similarly, a FBG can become a hydrogen sensor by coating it with a layerof Pallandium, which expands after absorbing hydrogen. The use of FBGsfor pressure sensing can be achieved where gratings are written intofibers with side cavities. This fiber structure converts side pressureinto axial stain at the core of the fiber. Because both the gratingpitch, Λ and refractive index η change with the temperature, the Braggwavelength of a FBG shifts with the temperature by approximately 1.7pm/° C. This, makes FBG a temperature sensor. However, no matter whichmeasurement that the FBG is adapted to detect, its output has to betemperature compensated if the original signal is below 20 Hz.

Charge-coupled devices (CCDs) have been widely accepted for solid stateimage sensing. However, image sensors based on complementary metal-oxidesemiconductor (CMOS) technology are becoming a major challenger to CCDsin the solid state imager field. All solid state image sensors comprisea one or two dimensional array of photosensitive elements or pixels,integrated on a semiconductor substrate. Each element produceselectronic charges in response to the photon energy incident on thepixel. Additional electronic circuitry is constructed on the samesubstrate to read out these charges as a voltage signal. The primarydifference between CCD and CMOS imagers is the way that these photoninduced charge packets are read out. In a CCD, charge packets arebrought to an output amplifier by coupling through a series potentialwells pixel by pixel in a sequential manner. Consequently, the CCD hasto read through every pixel in the entire array in order to find outphoton signal at a particular pixel. As a result, the frame rate of aCCD imager can not be very fast. Most industrial CCD cameras havestandard frame rate of 30 Hz or 25 Hz. CMOS technology can randomlyaccess information at a specific pixel individually, which makes itideal for some special applications such as missile tracking, where thearea of interest is only a small portion of the image and the event istoo fast to wait the entire image to be read out. Such randomaccessibility provides crucial performance enhancement for FBGinterrogation instrument herein described.

The CMOS technology is the standard in the making of semiconductorchips, which is used to fabricate almost more than 90% of VLSI circuits,from powerful microprocessors to memory chips. Because of this, CMOSimage sensors enjoy the key advantages over CCDs namely, low cost,miniaturization, lower power consumption and enhanced funtionality.

In one embodiment of the invention, a CMOS random access imager iscommercially available under the name FUGA image sensor series producedby IMEC and marketed by C-Cam Technologies. A FUGA15c has 512×512 squarepixels with 12.5 u.m. pitch; full digital operation, for both input andoutput signals, i.e. an 8 bit gray scale data at a specific pixel can beobtained by providing the chip with 9 bit X and Y coordinates; maximumpixel rate 5 MHz with 50 mW power dissipation: and logarithmic lightintensity to voltage conversion with a dynamic range over 50 dB.

Photobit Inc. based in California has plans to market a scientific grade(16 bit gray scale), random access. 512×512 pixel CMOS imager. NASA JetPropulsion Laboratory (JPL) in California has demonstrated such a chipat 2048×2048 format. Futhermore, there is another image sensortechnology termed charge-injection devices (CID), which is alsopotentially capable of random pixel access. Products with 512×512 pixelsand 16-bit gray scale have also been demonstrated. It is estimated thatthese products will be comnmercially available in two or three yearstime.

In target tracking applications, the precise position of a point objecton the image sensor has to be measured precisely. As shown in FIGS. 1C &1D, the intensity profile of such an object normally spreads over acluster of pixels on the imager. There are a number of interpolationalgorithms available, which make use of this intensity distribution tocalculate the center of the profile to sub-pixel precision. Among them,the centroid algorithm, noted above, is the most mature and versatilemethod because it simply calculates the “weight center” of the profile,thus does not have to know the shape of the profile in advance. Thealgorithm will work even when the profile is asymmetric, as long as itis stable. Using a two-dimensional (2D) centroid method, the preciseposition of the object along X coordinate (pixel rows), x_(c), iscalculated as:

x _(c)=Σ_(i) i(Σ_(j) g _(ij))/Σ_(i)Σ_(j) g _(ij)

where i, j is the column and raw number of a particular pixel in theimager, g is the gray scale, i.e. pixel output at this pixel and all thesum are within the cluster boundary.

The precision of this algorithm depends on the stability and shape ofthe intensity profile, the size of the pixel cluster used forcalculation and the pixel noise and uniformity of the imager. Aresolution of 1/83 pixel has been achieved in preliminaryl researchusing a low cost, industrial grade CCD camera and a 1/100 pixelresolution has been reported. Generally a larger spot ends to yield abetter resolution because of the averaging effect. However, study hasshown that interpolation resolution no longer improves when the spotbecomes larger than a particular cluster size, which is termed the“optimum cluster size”. Naturally, higher the imager quality (in termsof pixel noise and uniformity), the smaller the optimum cluster size.For industrial grade CCD camera with 9-bit gray scale, the optimumcluster was tested to be 4×4 pixels.

A smaller optimum cluster is advantageous because the processing speedof the interpolation (including pixel readout and computation) dependson the size of the spot cluster. According to the above equation, for aspot at size of K×K pixels, the processing time is approximatelyproportional to K². The processing speed can be increased by usingpixels for the calculation. An obvious option is to use only the one rowof the pixels (row J) that are near the center of the spot and to employan alternative one-dimensional (1D) centroid algorithm, which isexpressed as:

 x _(c) ≈x _(ci)=Σ_(i) ig _(ij)/Σ_(i) g _(ij)

Of course, the interpolation resolution will be reduced accordingly.

As depicted in FIG. 2, light from a broad band source is split into manysingle mode fibers (or termed “fiber channels”) by a star coupler. Alongeach fiber, there are a number of FBGs with pre-determined different,Bragg wavelengths. FBGs on different fibers, however, can have the samewavelengths. Light reflected from the FBGs in each fiber channel iscoupled into a down-lead fiber via a 1×2 coupler and sent to theinterrogation instrument. The instrument is basically a compact,two-dimensional (2D) optical fiber spectrometer. It can have twopossible configurations. The first configuration is shown in FIG. 2. Asecond configuration, schematically shown in FIG. 3 is termed“transmission configuration” because the light arriving at the imagesensor is transmitted through the FBGs instead of reflected by them. Inboth configurations, a 2D, random access image sensor is placed at theoutput port of a wavelength dispersion device. In addition, theend-faces of all the down-lead fibers are arranged to form a line Lpositioned at the input port. Furthermore, the image sensor chip ispositioned in such a way that its pixel columns (Y-axis) are parallel tothe grooves of the bulk grating in the spectrometer and to the line ofthe fiber end-faces. The digital output of the imager is sent to acomputer for processing.

The imaging system of the spectrometer separates light from differentfibers and distributes them along pixel columns of the 2D image sensor(Y-axis). Because of the effect of the bulk grating dispersion device,the light at different wavelengths will be diffracted onto differentdirections along X-axis, hence form bright spots at different positionsalong pixel rows of the image sensor. Therefore if the system has mfiber channels and n FBGs of different wavelengths along each fiber,there will be a mxn matrix of discrete spots on the image sensor array.A column of such spots in the matrix represents FBGs of the same orsimilar wavelength in different fiber channels, and a row representsdifferent FBGs along the same fiber. In other words, for each FBG, thespatial position of its fiber channel is encoded into the position alongY-axis of the imager while its wavelength is encoded along X-axis. Theprecise central wavelength of a FBG sensing node can therefore bedetected by locating the exact position of the associated spot alongX-axis of the imager pixel array.

Because of the random accessibility of the CMOS imager used, any FBG inthe network can be addressed in a truly random fashion by simply readout only the relevant pixels and calculate its centroid along X axis.This unique feature not only adds great flexibility in application butalso enables the system to utilize its resources efficiently resultingin quantum performance enhancement.

The Bragg wavelength of a FBG changes with the temperature by about 1.7pm/° C., translating to 2 με/° C. false strain signal at 830 nm region.For applications where the wanted signal is below 20 Hz, thistemperature induced variation has to be compensated. Although techniquesto separate temperature and strain induces signals at the same pint havebeen reported. They are either not accurate or require different sensorsthat are not based on FBG, thus are difficult to be integrated into theproposed network. Most practical applications do not require on-the-spottemperature compensation because the temperature field can beeffectively monitored using only a small number of dedicated temperaturesensing nodes. One method shown in FIG. 5A is to loosely house one ortwo fiber channels with FBGs in a small diameter (2 mm) tube and attachthe tube to the structure. In this way, the FBGs in the tube are onlyaffected by the temperature field and their outputs can be used tocompensation the temperature induced changes in other FBGs. This methodworks in many applications. However, in some applications, all sensingnodes have to be embedded into the host structure. Sometimes the tubemethod can not be used because of its relatively large diameter andincompatibility with the host material. According to the invention, adedicated temperature sensor is produced, the output of which is notaffected by structure strain even if it is embedded into the material.

As shown in FIG. 4A, an embeddable FBG temperature sensor 120 isfabricated by splicing a FBG made on a short 80 μm fiber 122 diameterfiber on to a 125 μm diameter fiber 124 and encapsulating it inside a250 μm silica tube 126. The free end 128 of the 80 μm fiber is cleavedinto an angle to reduce the unwanted Fresnal reflection. This sensorstructure ensures that the FBG will free from tensile strain even it isembedded into the host material. Of course, such a sensor can only beplaced at the end of each sensing fiber. So there will be maximum ndedicated temperature sensing nodes in the sensor network.

Alternatively, as shown in FIG. 4B an in line temperature compensatedstrain sensor 170 may be made by filling a fiber 172 having a pluralityof FBGs 174A, 174B in a tube 176. The FBG 174A is glued to the tube at178, or attach their two by other means. Meanwhile, FBG 174B is loose inthe tube. As a result, the output signal of FBG I 74B is independent ofthe stress and can be used to compensate the temperature inducedvariation in the signal of FBG 174A.

By properly selecting the groove density of the bulk grating, the widthof the 2D image sensor can just cover the spectral bandwidth full widthhaving maximum (FWHM) of the source. The spectral resolution Δλ of themeasurement of a FBG wavelength can then be expressed by the followingequation: Δλ=WE/C where W is the FWHM of the source, C is the totalnumber of the effective pixel columns in the image sensor chip and E isthe sub-pixel interpolation resolution. At the hardware level, theresolution can be enhanced by choosing a source with smaller FWHM and animager with larger number of pixel columns. However, there is a tradeoffwith the scale because a wider FWHM enables more FBGs to be incorporatedalong a fiber channel. Since the CMOS imager is based on silicon, it hasa characteristic photo sensitive region of 400 nm 1,000 nm withsensitivity peak at around 800 nm. Superluminescent diodes (SLD) withFWHMs as wide as 60 nm are available in this wavelength. However, mostSLDs in this region have a typical FWM of 15 nm, and they are much morepowerful than SLDs with wider FWHMs.

The tradeoff between the sub-pixel resolution, E, and the system speedlies in the pixel cluster size of the bright spot on imager produced bya FBG. A larger pixel cluster tends to improve E because of theaveraging effect but inevitably reduces the speed that a spot is readout and processed. Even the spot size has been chosen by hardwaredesign, the balancing point of this tradeoff can still be shifted to awide margin by intentionally read out only part of the pixels in thecluster for centroid calculation.

The maximum number of FBG based sensors that can be accommodated by thesystem depends on the number of FBGs multiplexible along a single fiberchannel and the number of fiber channels addressable by theinterrogation instrument. Referring back to the operating principles ofFBG based sensor systems, a FBG at Bragg wavelength λ_(b) moves within aspectral range of R_(c)λ_(b) in response to a R_(c) strain range. Manysystems can only provide a fixed spectral window for each FBG. The widthof these windows has to be at least R_(c)λ_(b) in order to prevent crosstalk between FBGs. The number of FBGs multiplexible along a fiberchannel, N, can thus be expressed as N=W/(R_(c)λ_(b)) where W is theFWHM of the source as before. Considering R_(c)=6000 με(3000 με), W=38nm and λ_(b)=830 nm, the maximum number of FBGs multiplexible along afiber channel is 7. However, it is highly desirable to multiplex moreFBG sensors along a fiber to make full use of the one-dimensional natureof the media. In practice, the strain at a particular position on thestructure can be estimated to certain accuracy. It is thereforereasonably possible to arrange the FBGs in such a way so that the straindifferent between two spectrally adjacent FBGs along the same fiber doesnot exceed a much smaller range (Δ R_(c)=1000 με). Because the spectralwindows in the proposed system can be flexibly shifted along the pixelrow, we can replace R_(c) in the above equation with Δ R_(c) and thenumber of FBGs multiplexible along a fiber can be increased to around45.

The maximum number of fiber channels that the system can accommodate, M,can be expressed as M=R/(2K) where R is the number of rows in the imagerand K is the spot cluster size. The separation between two rows of spotsis set as twice of the spot size to prevent cross talk between FBGs. ForK=5 and R=512, M=51. This means that an instrument with 512×512 imageris capable of multiplex a total of M×N=2295 FBGs.

The maximum sample rate to any FBG in the array is limited by thephotoreceptor time constant of the imager, which is inverse proportionalto the light density on the pixel. From the data provided by themanufacturer, the maximum sample rate can be expressed asf_(mx)=(d/8)×10⁵ (Hz) where d is the average light density within abright spot on the imager. In the invention, this density can beexpressed as:$d = {e\frac{P}{4\quad M}\frac{w}{W}\frac{1}{K^{2}p^{2}}}$

where P is the total output power of the source, w the spectral FWHM ofa FBG, p the pixel pitch and e the power efficiency of the entireoptical system, which includes the insertion loss at couplers, bulkgrating efficiency, and other features.

One 15 nm FWHM, single mode fiber pigtailed. SLD has a typical outputpower of 400 μW. P is therefore 800 μW over W=38 nm bandwidth for thecompound source combining the above two equations and assuming w=0.2 nm,K=5, p=12.6 μm and e=10%, maximum sample rate can be calculated to bef_(mx)=331.5/M(kHz). This equation represents a tradeoff between themaximum speed and scale. At the maximum scale (M=51), the maximum samplerate is 6.5 kHz.

The scale and speed figures presented above are exemplary maximums. Theactual achievable scale and speed of the system are most likely limitedby the system time budget. Although every FBG sensors in the proposedsystem can be addressed independently, they all complete for oneimportant system resource: time, which can be budgeted using thefollowing expression: Σf_(i)t_(i)≦1 (second) where subscript irepresents a particular FBG in the network, f is the sample rate forthat FBG, t is the time taken for one sample. While f must be smallerthan the maximum sample rate discussed above, the t is limited by pixelrate of the imager or the computation time to calculate the centroid.Because the data acquisition and calculation can be done in parallel,the slower of the two sets the limit.

The time taken to access a FBG, t, can be expressed as t=G/f_(p) wheref_(p) is the pixel read-out rate of the CMOS imager and G is the totalnumber of pixels the computer has to read for centroid calculation.Because the light spot is constantly moving along pixel row of theimager and the computer has to first locate the cluster before readingout relevant pixels. This is a necessary operation overhead. There aremany wavs to find out the location of the cluster with minimum overhead.The most conservative method would be to read out a row of pixels nearthe center of spots and locate clusters by a preset threshold. Theaverage number of pixels the computer has to read for locating a clusteris then R/N, where R is the number of pixel rows in the CMOS imager andN is the number of FBGs along a fiber channel. After the cluster islocated, an additional K(K−1) pixels have to be read before the centroidcalculation can be carried out if the 2D centroid algorithm is used. For1D centroid method, no more pixel read is required.

The FUGA15c has a pixel rate of 5 MHz and R=512. Assuming N=45 and K=5,we can calculate G=12 and the access time for one FBG in the proposedsystem as 2.4 μs or 6.4 μs using 1D or 2D centroid algorithm,respectively. Therefore, sensor access time is roughly in reverseproportion to the sub-pixel interpolation relolution.

The time to calculate the centroid of a 5×5 cluster using a high levellanguage program is approximately 7.6 μs on a Pentium 200 MHz PC withoutMMX. It is estimated that the processing speed can boost at leastfour-fold by program streamlining. With the fast advance ofmicroprocessor technology, there is plenty of computing power availableso that computation time can be easily brought down to a level wellbelow the FBG access time and will therefore not become the bottleneckof the system speed.

While there has been described are what are considered to be exemplaryembodiments of the invention. It will be apparent to those skilled inthe art that various changes and modifications may be made thereinwithout departing from the invention. It is intended in the attachedclaims to cover such changes and modifications as fall within the truespirit and scope of the invention.

We claim:
 1. A digital spatial and wavelength domain system formultiplexing fiber Bragg grating (FBG) sensors comprising: a pluralityof optical fibers, each including a plurality of fiber Bragg gratings(FBG) therein, each FBG having selective center wavelength beingvariable in accordance with strain for reflecting or transmitting lightat the corresponding center wavelength in accordance with the strainthereat; means for providing broad band light illumination for the FBGs;means for each optical fiber for carrying the light to a selectedlocation; a wavelength dispersion device responsive to the light fromeach of the fibers for wavelength separating the light in each saidfiber into the center wavelengths in accordance with the location ofeach fiber so that the selected location of each fiber and thewavelength separated light provides spatially independent signals foreach FBG in each optical fiber.
 2. A digital and spatial wavelengthdomain system comprising a plurality of optical fibers, each including aplurality of fiber Bragg gratings (FBGs), each having a centerwavelength; a broad band light source for illuminating each FBG; each ofsaid FBGs being operative for reflecting a portion of the light at thecenter wavelength corresponding thereto in accordance with a stressapplied to said fiber thereat; a wavelength dispersion deviceoperatively coupled to each fiber and responsive to the light forseparating the light in each said fiber into a sensible signal at thecorresponding wavelength for each FGB; and optically sensitive solidstate means spatially responsive to the sensible signal for producing anoutput for spatially separating the signals at each wavelength.
 3. Thedevice according to claim 2 wherein the wavelength dispersion devicecomprises a bulk grating.
 4. The device according to claim 3 wherein thegrating comprises a mirror lens having a focal plane and a gratingdisposed on a reflective surface thereof.
 5. The device according toclaim 4 wherein the grating includes parallel grooves formed in thereflective surface.
 6. The device according to claim 2 furthercomprising fiber means for carrying the light from the plurality of saidoptical fibers to said wavelength dispersion device, said fiber meanshaving output ends aligned in a linear array.
 7. The device according toclaim 2 wherein the optically sensitive means comprises a solid statesensing device including a plurality of pixels arranged in a twodimensional array.
 8. The device according to claim 7 wherein the pixelsare randomely accessible.
 9. The device of claim 2 wherein the imagingdevice includes a 2D array of pixels and wherein the wavelengthseparated light impinges on the array at selected pixel locations. 10.The device of claim 9 wherein the light from the impinging light forms aspot on the imaging device covering a plurality of pixels and furtherincluding processing means for sensing the light in each of said pixelsand weight averaging the light for determining a centroid of said spotcorresponding to the center wavelength thereof.
 11. The device of claim2 comprising at least one strain independent sensor means for each fiberfor providing a temperature calibration signal at a selected centerwavelength.
 12. The device of claim 11 wherein each of said plurality ofoptical fibers has a free end remote from the source, and the strainindependent sensor means is disposed at the free end of each of saidfibers.
 13. The device of claim 12 wherein the strain independent sensormeans is within the fiber.
 14. The device of claim 2 further comprisingmeans for at least one of detecting the center wavelength for eachwavelength separated signal in accordance with at least one of centroidweighting; curve fitting; and linear and higher order interpolation. 15.The device of claim 2 further comprising carrying means for carrying thelight to the wavelength dispersion device.
 16. The device of claim 15wherein the carrying means comprises a down-lead fiber for each opticalfiber.
 17. The device of claim 15 wherein the carrying means comprises afree end of the optical fibers.
 18. The device of claim 2 furthercomprising distributing means for distributing the light to each opticalfiber.
 19. The device of claim 18 wherein the distributing meanscomprises a star coupler.